Post on 04-Jan-2017
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PONDASI DANGKAL
MEKANIKA TANAH IIYULVI ZAIKA
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SYARAT- SYARAT PEMILIHAN PONDASI1. KEDALAMAN TANAH KERAS2. BEBAN YANG DITAHAN3. BIAYA YANG TERSEDIA4. PENGARUH-PENGARUH LAIN
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Combined footing
2 footings close
to each other
P1 close to property
line and P2 > P1
property line
P1 P2
If P2/P1 < 1/2, use strap combined footing
property line
If 1/2 < P2/P1 < 1
use trapezoidal footing
property line
P1 P2
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PONDASI TELAPAK MENERUS/ LAJUR
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SYARAT PONDASI DANGKAL
Kedalaman tanah pondasi kurang atau sama dengan lebarnya atau kedalaman (Terzaghi, 1943).
Teori lain, kedalaman pondasi dangkal 3-4 kali lebar pondasi.
Pondasi setempat harus memenuhi syarat-syarat:1. Stabilitas, aman terhadap keruntuhan geser 2. deformasi harus lebih kecil dari yang diizinkan.
Lapisan Tanah Keras
Df
B
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KERUNTUHAN PADA PONDASI DANGKALPADA TANAH PASIR PADAT DAN LEMPUNG KAKU
B
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MODEL KERUNTUHAN GESER
» keruntuhan umum geser» pasir padat Dr>67%
» lempung kaku NSPT >12
» Koruntuhan lokal» pasir atau
» pasir kelempungan» kurang padat (medium)
» 30%<Dr<67%
» Keruntuhan penetrasi » Pasir lepas
» Dr< 30%
Beban/luas
Penurunan
qu
Beban/luas
Penurunan
qu1
qu
Beban/luas
Penurunan
qu1
qu qu
Permukaan runtuh
Permukaan runtuh
Permukaan runtuh
(a)
(b)
(c)
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MODEL KERUNTUHAN BERDASARKAN RELATIF DENSITY
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Terzaghi Assumptions
• 1. Soil under footing is homogeneous and isotropic • 2. Soil surface is horizontally• 3. The base of footing is rough, to prevent the shear displacement.• 4 The foot is shallow foundation, i.e. the depth of foundation is less than
the width of foot…• Df ≤ B
• 3. Shear strength above the level of the base of footing is negligible. c = 0 above ( F.L ).
• 4. Consider only the surcharge which produced as uniform pressure • q = DF at foundation level.• 5. The load on foundation is vertical and uniform.• 6. The foot is long strip footing (pondasi lajur, B/L 0). • 7. =
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Shear failure happened on many stages
• I) Stage I: The soil in the elastic case and behave as the part of foundation it still that, and by increasing the load performed the region I which called active zone.
• II) Stage II: At this stage the foundation load effect on the active zone and neighboring soil so perform the region which called arc of logarithmic spiral zone.
• III) Stage III: By increasing the load performed the third part curve in which part the soil became in the passive case it make to resist the failure.
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Three components produced to resist the failure of soil.
I) (Pp)γ = Component produced by the weight of shear zone II, III.
II)(Pp)c = Component produced by the cohesive stress.
III) (Pp)q = Component produced by the surcharge q.
Pp = (Pp)γ + (Pp)c + (Pp)q
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DASAR PENURUNAN RUMUS TERZAGHI
• Kesetimbangan Gaya qu(2b)= -W+ 2Pp+f sin • W=(1/2)2b btan =b2tan• f=c DC= c. b/cos• Pp=(1/2) (b tan )2 K + c (b tan )Kc +q(b tan ) Kq• qu =c{tan (Kc +1)} +q(tan Kq) + B/2{(1/2) tan (K. tan -
1)}
Zona aktif
Zona pasif
Zona geser radial
quA B
J
PpPp
W
ff
B=2b
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PERSAMAAN DAYA DUKUNG TERZAGHIUNTUK PONDASI LAJUR/ MENERUS
12u c qq cN qN BN=
245cos2 2
2
a
Nq =
= e
( ) cot1-qNNc =
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TEORI DAYA DUKUNG TERZAGHI (1943)
Daya dukung:Pondasi lajur
Pondasi bujursangkar
Pondasi lingkaranUntuk keruntuhan geser lokal:
c’ = 2/3 c dan ’ = 2/3
qu
q=Df
B
Df
45-/2 45-/2 45-/2 45-/2 A C
D
EF
H G
J I
12u c qq cN qN BN=
1.3 0.4u c qq cN qN BN=
1.3 0.3u c qq cN qN BN=
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GRAFIK FAKTOR DAYA DUKUNG
General shear failureLocal shear failure
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RUMUS UMUM DAYA DUKUNG PONDASIMEYERHOF (1963)
• Fcs , Fqs , Fs = faktor bentuk• Fcd , Fqd , Fd = faktor kedalaman• Fci , Fqi , Fi = faktor inklinasi
12u c cs cd ci q qs qd qi s d iq cN F F F qN F F F BN F F F =
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FAKTOR PENGARUH MEYERHOF
• F. BENTUK F. KEDALAMAN F. INKLINASI
=
=
=
=
245tan
1.01
1.01
2.01
2
p
ps
pqs
pcs
K
LBKF
LBKF
LBKF
BDKF
BDKF
BDKF
pd
pqd
pcd
1.01
1.01
2.01
=
=
=
2
2
190
1
o
ci qi
i
F F
F
= = -
= -
Q
T
R
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DISTRIBUSI TEGANGAN TANAH AKIBAT TEG. VERTIKAL DAN MOMEN
P
M
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Tegangan kontak akibat beban vertikal dan momen
P
M
PMe=
e = B/6qmaxqmin
e < B/6qmaxqmin
e > B/6qmaxqmin
B
eR
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P
e = B/6qmaxqmin
e < B/6qmaxqmin
e > B/6qmaxqmin
e
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Stabilitas Daya Dukung
C
Tegangan kontak : tegangan akibat beban
Menentukan eksentrisitas DPT
XBeV
MX net -=
=2
;